Conventional CMOS image sensors can have a linear response to relatively low illumination and a logarithmic response to relatively high illumination, with a transition between the linear and logarithmic responses being referred to as a knee point (KNPT). As will be understood by those skilled in the art, an image sensor may be fully characterized when its linear sensitivity, logarithmic sensitivity (LOGS) and KNPT voltage are known. Because these parameters vary with process, transistor mismatch and temperature, etc., calibration is required. Without accurate calibration, a linear-logarithmic image sensor may introduce very large fixed pattern noise (FPN), which is typically not suitable for commercially viable applications. One conventional technique to inhibit FPN uses an external frame memory to store a sensor characteristic response. Based on this technique, a correction of the sensor readout for each pixel is performed after a full frame of image data is captured by an image signal processing (ISP) circuit and then modified by the sensor data stored in the external frame memory, which may have a relatively large capacity. Moreover, relatively complex ISP algorithms may be required to perform image sensor correction, which can be computationally expensive and time consuming.
A conventional linear-logarithmic image sensing apparatus is disclosed in U.S. Pat. No. 8,233,059 to Kamon et al., entitled “Image Sensing Apparatus.” In this apparatus, an image sensor is disclosed that generates an electrical signal commensurate with an amount of incident light. The sensor has a photoelectric conversion characteristic defined by a linear characteristic area where an electrical signal is outputted after being linearly converted in relation to the amount of incident light, and a logarithmic characteristic area where the electrical signal is outputted after being logarithmically converted in relation to the amount of incident light. An exposure evaluation value detector (EEVD) is also included, which detects an exposure evaluation value based on luminance information acquired from a subject in sensing an image of the subject. An exposure controller is further provided, which acquires a setting value for exposure based on the exposure evaluation value detected by the EEVD to thereby control exposure of the image sensing apparatus.